RELAY SWITCH CIRCUIT

Relays are widely used to protect electronic devices from short circuits these are electromechanical devices which uses electromagnet and it is used to close the movable contacts when it is in open position

Relays are used to control motors, heaters, or AC circuits which will draw a lot more electrical power. in relay switching circuits it uses transistors as the main switching device because the transistor can provide fast DC switching (on-off)

The main usage of the relay switch circuit is that if a low power electrical circuit is switched to high current or voltage both “on and off” relay switch circuit can control it.

while building circuits we need to use a transistor switch so that we can control high power devices such as motors, heating elements or AC circuits but these devices require large current than a single transistor can handle in that case we can use a relay switching circuit to do this


If the base voltage of the transistor is zero the transistor will cut-off and it will be an open switch and during this condition, there is no collector current flow and the relay coil will be de-energized because there is no current flow through the base and thus there is no current flow through the relay coil. If a large current flows to the base to saturate the NPN transistor the current flowing from the base to emitter will control the large relay current that flows through the collector to emitter


The PNP relay switches circuit is the opposite to the NPN relay switching circuit if the base is forward biased with the voltage more negative than the emitter then the load current will flow from the emitter to the collector. Both the base and collector must be negative with respect to the emitter then only the relay load current will flow through the emitter to the collector. If the Vin is high then the PNP transistor is switched off and so the relay coil will do the same if the Vin is low the base voltage will be less than the emitter voltage and the PNP transistor will turn On the resistor value in the base sets the base current and it will set the collector current that drives the relay coil